As is known, it is quite common to produce tubing by cold-drawing techniques wherein the wall thickness of the tubing is constant throughout its length. Both the inner and outer diameters of a tube can be reduced or expanded together; however it is uncommon to provide tubing in which the wall thickness varies along its length. This is true particularly in the case of tubing of constant outer diameter but a wall thickness which is greater at the ends of the finished tube than in the portion of the tube intermediate its ends. This is for the reason that such tubing is normally formed as it passes through a die over a mandrel. There is no way of pulling the tube over a mandrel in a drawbench, for example, if it is desired to produce a tube with an inner diameter smaller than that of the mandrel at the end through which the mandrel must exit.
While it is uncommon to produce tubing of varying wall thickness, there are applications for tubing of constant outer diameter in which the gage or thickness of the tube wall is greater at its ends than at its intermediate portion. One such application is a bicycle frame wherein the wall thickness is preferably light intermediate the ends of a tubular frame member to facilitate light weight and a degree of flexibility, but wherein the wall thickness at the ends of the frame members must be greater to facilitate connection (e.g., by welding) to other frame members.
In the past, methods have been devised for producing tubing of variable internal diameter of the type described above; however these methods are cumbersome, expensive and not altogether satisfactory.